1. Field of the Invention
The present invention relates generally to tarp systems including fabric or other flexible material and associated metal hardware for covering an open top truck box, railcar, other vehicle or stationary container. More particularly, the invention relates to mechanisms for facilitating the rolling or unrolling of flexible material between a rolled up condition and the unfurled condition to cover an opening.
2. Description of the Prior Art
Various types of truck, trailer, railcar and moveable and stationary container bodies are designed to be loaded through an open top. Such bodies are typically used to haul or store hardware, equipment, produce, grain, stone, earth or refuse. It is desirable to use tarpaulins or other coverings to close the open tops of such containers. The tarpaulins serve to shelter the truck, trailer or container contents against the elements and to maintain the contents in the body.
Several systems are known for reversibly covering the open tops of vehicles, boxes and containers with tarpaulins. Typically, the covering is unrolled from a long tube or bar by manipulation of the associated metal hardware. The roll tube or roll bar is moved between an open, rolled-up position in which the covering is wrapped around the tube, exposing the interior of the container, and a closed, unfurled position in which the covering is spread over the top of the container.
Various ways have been employed to move the tube across the opening and to secure the free end near one edge of the vehicle, box or container top. The tube usually is rotated either manually or by an electric motor. These systems tend to use end caps and metal bows spanning the width of the opening to support the covering along the length of the open top.
An example of such a system is shown in U.S. Pat. No. Re. 31,746 issued to Dimmer et al., which is incorporated by reference herein in its entirety. As shown in the patent, a covering is attached along one lengthwise edge to a tube. One end of the tube is connected by a universal joint to a crank. The universal joint is reversibly attached to the roll tube by a spline and pin or similar mechanism.
An operator at ground level can turn the crank causing the tube to roll over the top of the container, such that the covering rolls up lengthwise on the tube. As a result, the material will uncover or cover the top of the trailer or container. In this arrangement, the end of the roll tube to which the universal joint and crank arm are attached can be described as the active end because the torque or turning force is applied there. Conversely, the opposite end can be described as the passive end because the torque is applied indirectly through the length of the tube.
A feature of the system shown in U.S. Pat. No. Re. 31,746 (and the commercial embodiment thereof) is an elastic cord (item 78) attached at one end to a narrower plastic tube within the roll tube or roll bar and at the other end to a forward edge of the latch plate (or, alternatively, to the body itself). (See U.S. Pat. No. Re. 31,746, column 4, lines 49-66.) When the roll tube is rolled across the top, the elastic cord pulls the passive end of the tube toward the latching side of the container to tension the end of the roll bar opposite the direct rolling force provided on the active end by the crank. Generally, the tensioning force is greatest when the roll bar is in the open position; this force decreases as the roll bar is moved closer to the latching side.
Another example of a tarp system is disclosed in U.S. Pat. No. 5,487,584, also incorporated by reference herein in its entirety.
Although the use of a resilient cord is intended to assist movement of the passive end of the roll tube, it has some disadvantages. For example, the resilient cord applies variable force as the roll bar moves across the opening. In essence, as the cord is stretched it applies an increasing force at one end of the bar in a different direction than the turning/rolling force applied directly by the crank arm at the opposite end. Also, the elastic cord stretches out over time, loses its pulling force, and is subject to being cut or breaking as a result of overuse or exposure to the elements.
During the opening operation, an imbalance of forces applied at the opposite ends of the roll bar can cause the covering material to be wrapped more tightly around one end of the bar or tube than the material on the other end. This will result in one end progressing more slowly than the other end. As a result, one end of the roll tube tends to advance faster than the other end during opening or closing operations, occasionally resulting in stationary unwinding of the covering from the bar at one end. Such uneven rolling may cause the operator to try to use the crank arm to slide, push or pull one end of the roll bar and associated covering material to advance it to a position even with the other end.
It is an object of the invention to provide an assist mechanism for a rolling tarp system whereby a more even winding or unwinding of the covering on the roll bar is achieved. It is another object of the invention to provide an apparatus where uneven rolling or unrolling is less likely to occur or is avoided altogether. It is a further object of the invention to provide a mechanism that assists with even progression of the two roll bar ends as the bar is rolled across the opening of a vehicle, box or container. Still further it is desirable to provide an assist mechanism that may be retrofitted readily to existing covering apparatus to improve the rolling operation. It is yet another objection of the invention to eliminate the disadvantages of use of the elastic cord, such as fraying, breaking, or loss of elasticity.
The present invention is an improvement for a tarp system for an open top container, wherein the tarp has first and second longitudinal edges and the container has first and second longitudinal sides, the tarp being attached on one longitudinal edge to the first longitudinal side of the container and on the other longitudinal edge to a roll bar. The roll bar is caused to roll by a force applied on at least one end of the bar. The improvement relates to a mechanism for assisting movement of the roll bar across the opening of the container.
The improvement comprises a roll assist apparatus including at least one reel fixedly attached to one end of the roll bar and a constant force spring having a first end fixedly attached to the reel and a second end attached to the second longitudinal side of the container. The roll bar is adapted to roll across the top of the container between the first and second longitudinal sides. When the roll bar is at the first side of the container, the tarp is rolled around the roll bar so that the top opening of the container is uncovered, and the constant force spring is extended. As the roll bar rolls toward the second longitudinal side of the container, the tarp unfurls to cover the top opening side of the container, and the constant force spring winds into a coil around the reel.
The constant force spring, by its nature, is biased to wind up on itself. This force tends to pull the tarp toward the unfurled position. Unlike a stretch cord, which has an increasing force as the roll bar moves away from the unfurled position, the force applied to the roll bar by the constant force spring is relatively constant over its length. The spring also is highly durable and unstretchable.